Automobile tires are required to have specific properties such that, firstly, automobiles exhibit a low fuel consumption rate, i.e., a long distance travel can be obtained with a low fuel consumption; secondly, the tires exhibit good wet grip property, i.e., braking distance on a wet road is short; and thirdly, the tires have good abrasion resistance, i.e., the surface of tire is not readily abraded.
In recent years, a serious view has been taken of environmental protection and saving in resources consumption, and thus, automobile tires with a low fuel consumption rate are severely required. To provide a tire with satisfactorily low fuel consumption rate, it is generally effective to use a rubber material capable of giving a rubber vulcanizate exhibiting a low heat build-up, i.e., in which heat is not readily built up.
As rubbers for tires, natural rubber (NR), polybutadiene (BR), polyisoprene (IR) and a styrene-butadiene copolymer rubber (SBR) are popularly used. Usually, rubber compositions comprising these rubbers having carbon black incorporated therein are used as a rubber material for tire. However, such rubber compositions containing carbon black are unsatisfactory in low heat build-up.
Incorporation of silica instead of carbon black in the rubber compositions has been proposed to improve the low heat build-up. However, silica-incorporated rubber compositions have a poor abrasion resistance as compared with carbon black-incorporated rubber compositions. This would be due to the fact that silica has poor affinity for rubber as compared with carbon black, and a satisfactory reinforcing effect cannot be obtained with silica.
To improve the low heat build-up, a proposal has been made wherein a coupled polymer is used as a rubber composition, which polymer is prepared by coupling living polymers having an activated terminal, prepared by polymerization using an organolithium catalyst, with a coupling agent such as tin tetrachloride. For example, a coupled polymer has been proposed in Japanese Unexamined Patent Publication No. S57-55912 which is comprised of styrene units and butadiene units, and has a bond between a butadienyl group and at least one metal selected from silicon, germanium, tin and lead, and has a high vinyl bond content. This coupled polymer gives a rubber composition exhibiting a somewhat improved low heat build-up when carbon black is incorporated therein, but, gives a rubber composition exhibiting only very slightly improved low heat build-up and a poor abrasion resistance when silica is incorporated therein.
Another proposal has been made in Japanese Unexamined Patent Publication No. H7-292161 which discloses a rubber composition for use in a tire tread, which rubber composition comprises an oil-extended rubber comprising a specific styrene-butadiene copolymer rubber prepared by a solution copolymerization using an organolithium catalyst, and further comprises silica, carbon black, a silane coupling agent and a vulcanizing agent. This rubber composition exhibits improved low heat build-up and improved abrasion resistance as compared with a rubber composition containing only carbon black as a reinforcer.
Still another proposal has been made in International Publication No. WO 96/30419, which discloses a styrene-butadiene-isoprene random copolymer rubber containing 0.5 to 10% by weight of isoprene. This copolymer rubber gives a rubber composition exhibiting, when silica is incorporated therein, good low heat build-up and good abrasion resistance as compared with a rubber composition having no isoprene units, but exhibiting only slightly improved balance among the low heat build-up, wet grip property and abrasion resistance, as compared with a rubber composition comprising a butadiene-styrene copolymer rubber having no isoprene units.